There is a method to perform synchronous control between circuits by: a certain circuit transmitting a synchronization clock; and the other circuits operating based on the synchronization clock. Also, there is a method to improve reliability in preparation for a failure and the like of an active system circuit by: multiplexing circuits for transmitting a synchronization clock; making one circuit among those multiplexed circuits activate a clock transmission function as an active system; and making the remaining circuits be stand-by systems. In such multiplexed circuit configuration, when an active system circuit has a failure or when a preventive maintenance replacement is carried out to the active system circuit, the function of the active system circuit is made to be stopped, and one of stand-by system circuits is made to function as an active system circuit newly.
In a structure in which clock transmission circuits are multiplexed, it is necessary not to lost synchronization between subordinate circuits even if a circuit to transmit a clock is switched.
In patent document 1 (Japanese Patent Application Laid-Open No. 2013-201743), there is disclosed a clock switching device which can prevent synchronization between circuits from being lost in association with switching of a clock transmission circuit. In a clock switching device in patent document 1, synchronization between circuits is achieved by adjusting the phase of a stand-by system clock to the phase of the active system clock in advance so that a transmission clock after switching a clock transmission circuit can be the same phase with the clock before the switching.
Generally, when a synchronizing operation is performed between a plurality of reception circuits based on a synchronization clock outputted from a multiplexed clock transmission circuit, there is a problem that a difference in received clocks occurs between the reception circuits. It is because a path difference to the clock transmission circuit exists between each reception circuit.
For example, when switching is carried out at timing when an active system clock and a stand-by system clock is the same phase mostly and, in addition, at timing of a clock rise, there is a case where a short pulse occurs in addition to a usual clock. At that time, there is a case where a reception circuit with a short path length receives clocks increased by one pulse unnecessarily, and a reception circuit with a long path length receives clocks without increase of one pulse because the pulse is dulled. As a result, a difference occurs to received clocks between the reception circuit with the short path length and the reception circuit with the long path length.
In addition, there is an individual difference in frequency characteristics of oscillators included in clock transmission circuits. Therefore, in a multiplex structure in which an active system clock and a stand-by system clock are generated from separate oscillators, even if the clocks are of the same phase at a certain time point, phases may shift a little at a next time point. When clock switching is carried out in a state that phases are shifted a little, there is a case where a short pulse occurs in addition to a usual clock. At that time, according to a path difference from a clock transmission circuit to each subordinate circuit, there exist a circuit which recognizes the short pulse and a circuit which does not recognize it. As a result, at the time of clock switching, there are a circuit which receives usual clocks and a circuit which receives clocks that are more than usual by one pulse unnecessarily, resulting in a synchronizing operation between the circuits based on a received clock not being performed correctly.
According to a clock switching circuit in patent document 1, loss of synchronization between circuits associated with clock switching can be prevented by using an identical clock supply part. However, there is a problem that, even by a clock switching circuit in patent document 1, loss of synchronization between circuits caused by an individual difference of frequency characteristics of an oscillator and a path difference cannot be prevented in a structure to achieve multiplexing using separate clock supply parts.